/Engine/Source/Runtime/Engine/Classes/Components/SkeletalMeshComponent.h
Takes sorted array Base and then adds any elements from sorted array Insert which is missing from it, preserving order. this assumes both arrays are sorted and contain unique bone indices.
static void MergeInBoneIndexArrays ( [TArray](API\Runtime\Core\Containers\TArray)< FBoneIndexType > & BaseArray, const [TArray](API\Runtime\Core\Containers\TArray)< FBoneIndexType > & InsertArray )